Going south, the route starts at the Johns Cross roundabout near Mountfield. From this point the current route of the A21 continues south-east through Vinehall Street, Whatlington and Kent Street, while the A2100 goes more or less due south.

It initially travels through woods, and crosses the Hastings railway line at a level crossing. It later forms the main street of Battle, and then turns south-east towards Hastings, eventually meeting the A21 and the A28 at Baldslow.